Poor service

MTN relayed to me that I was due for an update, and finally after six months I went to the store. A very nice young man, who incidentally does not work there any more, helped me. I upgraded and specifically told the person that I do not want a capped contract, because if I need to phone,I must have airtime. He assured me that this would be the case. Unfortunately now, for the third month in a row, my phone keeps telling me that i have insufficient airtime. This is despite the extreme efforts of my local store's very helpful and friendly staff to rectify the situation. MTN offices phoned today to tell my local store that the whole issue has been resolved, and they contacted me to tell me. Unfortunately that was not the case, and my phone is still telling me insufficient airtime. This is extremely difficult, as I am principal at a school and I cannot phone anyone!